1. Field of the Invention
This invention relates generally to the field of blood coagulation analysis and more specifically to a method and apparatus for quantifying blood coagulation factors.
2. Related Art
In the field of clinical laboratory medicine, in the area of hemostasis, it is often desired to monitor the coagulation process of blood in order to determine how various factors in a blood sample impact the clotting time of the sample. Blood coagulation is a complicated process involving a large number of blood components including fibrinogen and prothrombin. Prothrombin is a protein that is activated by an enzyme complex formed on the site of an injury to produce thrombin. Thrombin cleaves fibrinogen molecules prior to fibrin polymerization to produce fibrin molecules that aggregate and form a blood clot. By monitoring components such as fibrinogen and prothrombin levels within the blood, a physician may acquire meaningful data concerning a patient""s blood clotting abilities or other clinical conditions.
The proteins that are involved in the blood clotting process are commonly referred to as factors. The factors are numbered I-XIII, and reference to a factor by its number identifies the corresponding protein to those of skill in the art. The activation of prothrombin occurs as a result of the action of blood clotting factor Xa, which is formed by the activation of Factor X during proteolysis. There are two molecular pathways that lead to the activation of factor X to give Xa, generally referred to as the extrinsic and intrinsic pathways for blood clotting. The extrinsic pathway utilizes only a tissue factor specific to the injured membrane while the intrinsic pathway utilizes only factors internal to the circulating blood. Both of these pathways originate with the interaction of enzymes involved in the blood clotting process with surface proteins and phospholipids.
Various tests have been introduced to measure the coagulation process in both the extrinsic and intrinsic pathways of a patients blood sample. For example, the Activated Partial Thromboplastin Time (APTT) Test measures the coagulation factors of the intrinsic pathway. These factors include Factors XII, XI, X, IX, VIII, V, II and I which may be abnormal due to heredity, illness, or the effects of heparin therapy. Thus, the APTT test is useful as a presurgical screen and for monitoring heparin therapy. Similarly, the testing of the fibrinogen polymerization rate using a Thrombin Time (TT) test or a quantitative fibrinogen test provides useful diagnostic data.
Substantial efforts have been made to measure the level of the clotting factors during coagulation, particularly that of fibrinogen because it is one of the key factors in the clotting process. Most methodologies rely upon either immunologic or clotting techniques. Although immunologic techniques are generally capable of precisely defining the levels of the various components within the blood stream, they are often incapable of distinguishing between functional and non-functional forms of the components within the blood stream. Accordingly, immunologic techniques are felt to be less accurate at measuring blood clotting factors than clotting techniques.
Clotting techniques use coagulation timers to measure the elapsed time between the addition of a coagulation stimulating reagent to a blood sample and the onset of coagulation or fibrin polymerization. Coagulation instruments have been used for performing a variety of clinical chemistry tests such as a Prothrombin Time (PT) or Quick Test; an Activated Partial Thromboplastin Time (APTT); and a fibrinogen assay such as a Clauss Test.
Typically most clot detection instruments detect the formation of a clot in a patient""s blood sample by monitoring either optical turbidity or mechanical properties of the patient""s blood sample. The Fiberometer (manufactured by Beckton-Dickinson Microbiology Systems of Sparks, Maryland) is an example of an instrument which determines coagulation time by mechanical means. In such an instrument a timer runs only if an electrical switch contact which is immersed in the sample can be opened and closed repetitively. When the fibrin clot immobilizes the switch contact the timer is forced to stop. Other similar mechanical methods utilize magnetic fields to move metal balls or rods immersed in the clotting test sample by which means a timer is stopped when the fibrin clot immobilizes the device. Coagulation instruments based an optical methods tend to measure the xe2x80x9consetxe2x80x9d of clotting, rather than the formation of a clot as determined by the above described mechanical methods. Optical turbidity may be determined by measuring the decrease in light transmission through a blood sample due to the formation of a clot. Basically optical coagulation instruments tend to detect relatively short strands of fibrin prior to the clot point.
According to one prior art method of determining the chemical factors present in the patients blood sample, the clot time of the patients blood sample, measured using either the optical turbidity or mechanical technique, is compared against a calibration curve. The calibration curve predicts the probable concentrations of factors for the patients blood sample according to the measured clot time. The calibration curve is generated in response to a series of clot time tests, performed on various dilutions of a control plasma sample. Thus, when an unknown patient sample is tested, the sample""s clot time is converted to a factor concentration using the calibration curve.
One problem inherent in the prior art techniques for determining factor concentration is that the determinations are made at a qualitative, rather than a quantitative level. As such, the accuracy of the results is often suboptimal. Indeed, it is difficult to determine multiple factor concentrations from a single clot time measurement, at least in part because it is difficult to consistently identify the intended spot signifying clot time on the measurement waveform.
Traditional clinical chemistry equipment that measures such things as glucose do so by very quantitative means which readily relate to simple chemical reactions. In contrast, blood coagulation is a highly complex chain of chemical reactions. Coagulation timers seek to measure a parameter which correlates poorly to the chemistry which causes coagulation. They have therefore conventionally employed a qualitative measure. The instruments measure a gelling of a sample. That the gel forms is deterministic. The strength of the gel formed is also deterministic. However, the relationship between changes in physical properties over time as the gel forms and the initial chemistry is not highly correlated. Yet, conventional methods measure such physical properties. Optical methods repeatedly bombard the gel with beams of light and obtain results that indicate the operation of the chemistry. However, although these methods indirectly measure the chemistry of interest, they do not directly measure the chemistry. That indirection is one weakness of the prior art because formation of the clot never occurs exactly the same way, even in identical samples.
Thus, a problem exists as to how to create a good analytic model that translates a poorly correlated measured parameter event into a meaningful chemistry or chemical assay. While the above described hardware implementations generally provide acceptable results, such qualitative analysis lacks accuracy, as explained above, and yields only a narrow range of useful information.
According to one aspect of the invention, a non-linear logistic equation is provided that precisely matches the optical density, transmission or turbidity versus time of a clotting sample. The equation may be curve fit to a signal representing the optical density and/or turbidity of a blood sample. The equation provides as an output a set of coefficients which can be used directly, or converted by calibration curves or trained neural networks, to identify physical characteristics, such as clot time and fibrinogen concentration, of the blood sample. By using such an equation, quantitative tools for identifying the underlying chemistry of the blood clotting process may be provided. The coefficients of the logistic may be advantageously translated to identify physical characteristics of the blood sample through the use of multi-variate calibration curves or trained neural networks. Optimization logic may also advantageously be executed before the multi-variate calibration or use of a neural network to improve the performance of the inference engine by filtering exception conditions.
According to another aspect of the invention, a low level oscillatory signal is extracted from a clotting signal of a waveform. The clotting signal may be, for example, a signal of optical density, transmission, or turbidity versus time. The low level oscillatory signal may be used in a variety of manners. First, it may be used to compute a clot time by such means as, but not limited to, Fourier, FFT, wavelet, peak and minimum search, or by hardware means such as phased lock loops. Second, the low level oscillatory signal may be used to determine characteristics of the sample including kinetic reaction rates, reactant concentrations, abnormalities, or fibrin strand mass length ratio. The determination of the characteristics may be used to correct fibrinogen estimation through analysis of oscillation frequencies or time series profiles of the oscillation frequencies of a sample. Third, the extracted low level oscillatory signal may be used to compute initial starting coefficients for the logistic equation to thereby enhance the precision of the curve fitting operation.
According to another aspect of the invention, knowledge as to the underlying behavior of the clotting chemistry may be used to determine where to steer the bounds of the curve fitter to obtain initial estimates of the coefficients for the logistic. During the curve fitting process, for both the logistic and the polynomial equations, underlying knowledge regarding the chaotic characteristics of the clot waveform may be used to better weight individual data points to be curve fit, or adjust such things as sum of squares error. By using such a technique, the chaotic parts of the signal may be either filtered out or highlighted, depending upon the needs of the consumer.
According to another aspect of the invention, an apparatus for verifying the functionality of a clotting analyzer includes clot simulation means, coupled to the clotting analyzer for providing a simulated clotting signal, wherein the simulated clotting signal is provided using the non-linear function describing the entire clotting waveform. With such an arrangement, a straightforward method of providing a simulated clot waveform for testing clotting analyzer apparatus is provided that does not require storage of recorded clot waveforms.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.